Method and apparatus for LWD shear velocity measurement

ABSTRACT

The logging tool of this invention includes a transmitter conveyed on a drilling collar for exciting a quadrupole signal in a borehole being drilled by a drill bit and a receiver for receiving the signal. The transmitter is operated at a frequency below the cut-off frequency of the quadrupole collar mode. The received signal consists primarily of the formation quadrupole mode which, at low frequencies, has a velocity that approaches the formation shear velocity. The transmitter, in one embodiment, consists of eight equal sectors of a piezoelectric cylinder mounted on the rim of the drilling collar. The value of the cut-off frequency is primarily dependent on the thickness of the drilling collar. Alternatively, the transmitter may be operated to produce both the collar mode and the formation mode and a processor may be used to filter out the collar mode.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/045,263 filed on Nov. 7, 2001 now U.S. Pat No. 6,850,168, with apriority claim to U.S. Provisional Patent Application Ser. No.60/248,097 filed on Nov. 13, 2000. This application is also related toU.S. patent application Ser. No. 09/590,906 now U.S. Pat. No. 6,614,360,having the same assignee as the present application and the contents ofwhich are fully incorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to a method and apparatus for measuringand processing a characteristic of subsurface earth formationspenetrated by a borehole. More specifically this invention relates to amethod and apparatus for measuring and processing an acousticcharacteristic such as formation shear wave velocity of subsurface sonicwaves after these waves traverse earth formations adjoining a boreholeor passing through a portion of the subsurface.

BACKGROUND OF THE INVENTION

To obtain hydrocarbons such as oil and gas, wellbores (also referred toas the boreholes) are drilled by rotating a drill bit attached at theend of a drilling assembly generally referred to as the “bottom holeassembly” (BHA) or the “drilling assembly.” The wellbore path of suchwells is carefully planned prior to drilling such wellbores utilizingseismic maps of the earth's subsurface and well data from previouslydrilled wellbores in the associated oil fields. Due to the very highcost of drilling such wellbores and the need to minimize time actuallyspent drilling and wireline logging wells, it is essential to gain asmuch information as possible during drilling of the wellbores.Information about downhole conditions and materials may be acquired withwireline tools or bottom hole assemblies (BHA). Wireline tools aregenerally used after a wellbore is drilled, bottom hole assemblies maybe used while the well is being drilled as part of the drilling string.Downhole wellbore information acquired from BHA components may beutilized, among other things, to monitor and adjust the drillingdirection of the wellbores or to detect the presence of geologicformations and hydrocarbons.

In logging while drilling through an earth formation, it is desirable tomeasure formation shear wave velocity. The shear wave velocity of earthformations provides information important for exploration and productionof oil and gas from the formation. The shear wave velocity profileenables the conversion of seismic shear wave time sections to depthsections and is utilized in the interpretation of seismic wave amplitudevariation versus detector offset. The ratio between the shear wavevelocity and the compressional wave velocity is closely related to therock lithology and is related to hydrocarbon saturation. Shear wavevelocity is also used to evaluate the mechanical properties of theformation in reservoir engineering applications.

Because of the importance of earth formation shear velocity, variousmethods have been developed to measure it. In conventional wirelinelogging using a monopole acoustic tool, the shear velocity can bemeasured from the shear wave refracted along the borehole wall if theformation shear wave velocity is greater than the borehole fluidacoustic velocity. A formation that has a shear wave velocity fasterthan the borehole fluid is called a ‘fast formation.’ However, in aformation where the shear velocity is slower than borehole fluidvelocity, a ‘slow formation,’ the shear wave can no longer refract alongthe borehole wall, and the shear velocity cannot be directly measuredfrom monopole logging. Because of the need to measure shear velocity inslow formations, especially in the soft sediments of deep-waterreservoirs, dipole acoustic logging tools were developed. The dipoletool induces and measures the bending or flexural wave motion in theformation. In a sufficiently low frequency range (1–3 kHz), the flexuralwave travels at the shear velocity of the formation, regardless whetherthe formation is fast or slow. This allows for direct measurement offormation shear velocity using the dipole acoustic tool. Dipole acousticlogging is now a mature technology with worldwide commercialapplications.

A viable technique for shear wave velocity measurement is using thequadrupole shear waves. A quadrupole acoustic tool induces and measuresthe quadrupole shear wave in the formation. The low-frequency portion ofthe wave travels at the formation shear wave velocity, allowing fordirect shear velocity measurement from the quadrupole wave. Although thequadrupole shear wave has been extensively studied theoretically and awireline quadrupole-logging tool was also proposed (Winbow et al., 1991in U.S. Pat. No. 5,027,331), this technology has not yet beencommercially applied to the oil and gas industry. This is largelybecause the wide acceptance and success of the dipole shear wavetechnology have fulfilled the needs for measuring shear velocity in slowformations.

The acoustic Logging-While-Drilling (LWD) technology has been developedin recent years out of the needs for saving rig-time and for real-timeapplications such as geosteering and pore pressure determination, amongothers. The LWD acoustic technology is aimed at measuring thecompressional- and shear-wave velocities of an earth formation duringdrilling. This technology has been successful in the measurement ofcompressional wave velocity of earth formations. The need fordetermining the shear wave velocity in slow formations calls for furtherdevelopment of the technology for shear wave measurement capability.Because of the popularity and success of the dipole shear wavetechnology in wireline logging, this technology is naturally extended tothe LWD situation and a LWD dipole acoustic tool has been built andoffered for commercial applications.

As discussed later in the application, the application of the dipoleacoustic technology to LWD has a serious drawback caused by the presenceof the drilling collar with BHA that occupies a large part of theborehole. The drawback is that the formation dipole shear wave travelingalong the borehole is severely contaminated by the dipole wave travelingin the collar. There is a need for a method of determination of shearwave velocities of earth formations that is relatively robust in thepresence of tool mode waves propagating along the drill collar. The needis particularly acute in situations where the formation shear velocityis less than the velocity of propagation of compressional waves inborehole fluids. The present invention satisfies this need and,additionally, also provides a method for determination of shearvelocities in “fast” formations where the shear velocity exceeds thecompressional velocity of borehole fluids.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method of determiningformation shear properties in a Logging while Drilling environment. In apreferred embodiment of the invention, a transmitter is operated at afrequency below a cut off frequency of a signal propagating through thedrill collar so that the signal received at receiver is uncontaminatedby the collar mode. The cutoff frequency is determined by the thicknessof the drill collar. In a preferred embodiment of the invention, thetransmitter is a quadrupole transmitter. The quadrupole mode in a collarhas a cutoff frequency that is higher than the quadrupole mode in theformation, so that operating a quadrupole transmitter below this cutofffrequency gives a signal relatively uncontaminated by the tool mode. Ina fast formation where the formation shear velocity exceeds thecompressional velocity of the borehole fluid, higher order quadrupolemodes may be analyzed to determine the formation shear velocity.

In an alternate embodiment of the invention, the transmitter is operatedwithout regard to the cutoff frequency and the received signals arelow-pass filtered to reject the tool mode.

In a preferred embodiment, the transmitter comprises eight azimuthallysegmented elements. The receiver may be a quadrupole receiver or adipole receiver. When a dipole receiver is used, the quadrupoletransmitter is operated twice with different polarizations and the tworeceived signals are then processed to give a received quadrupole signal

The receivers are preferably located between the transmitter and thedrillbit. Noise generated by the drillbit is received at the receiverswith a direction of propagation opposite that of signals produced by thetransmitter. The receivers may be operated as a phased array to rejectthe noise generated by the drillbit.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are believed to be characteristic of theinvention, both as to organization and methods of operation, togetherwith the objects and advantages thereof, will be better understood fromthe following detailed description and the drawings wherein theinvention is illustrated by way of example for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention:

FIG. 1A shows a schematic diagram of a drilling system that employs theapparatus of the current invention in a logging-while-drilling (LWD)embodiment.

FIG. 1B illustrates a LWD tool on a drill collar.

FIG. 2 illustrates velocity dispersion curves for formation anddrill-collar dipole modes.

FIG. 3 illustrates dipole acoustic waveforms in the presence of adrilling collar.

FIG. 4A shows is a schematic illustration of the field for a quadrupolewave.

FIG. 4B illustrates velocity dispersion curves for formation anddrill-collar quadrupole modes.

FIG. 5A illustrates wavefield snapshots of acoustic energy near theborehole, where radial particle velocity associated with the quadrupolewave motion is displayed.

FIG. 5B illustrates quadrupole waveforms at various offsets.

FIG. 6A illustrates wavefield snapshots of acoustic energy near theborehole, where radial particle velocity associated with the quadrupolewave motion in a thin-collared pipe is displayed.

FIG. 6B shows time domain displays of the data in FIG. 6A.

FIG. 7 illustrates a simplified perspective view of a sourcecross-section in conjunction with the drilling collar.

FIG. 8 illustrates a detailed cross-sectional view of a segmented ringtransmitting implementation of a source.

FIG. 9 illustrates a detailed cross-sectional view of acompartmentalized segmented transmitting implementation of a source.

FIG. 10 is a perspective view of the sensor layout around the collar.

FIG. 11 illustrates a detailed cross-sectional view of a segmented ringreceiving sensor implementation.

FIG. 12 illustrates a detailed cross-sectional view of acompartmentalized segmented receiving sensor implementation.

FIGS. 13A and 13B show the preset invention using two receiver elementsfor detection of quadrupole signals.

FIG. 14 illustrates a side view of a portion of the device of FIG. 12.

FIG. 15 shows the higher order quadrupole modes in a fast formation.

FIG. 16 shows the equivalence of a quadrupole to a pair of dipoles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a method, system and apparatus for measuringshear wave formation velocities while a well is being drilled. To theextent that the following description is specific to a particularembodiment or a particular use of the invention, this is intended to beillustrative and is not to be construed as limiting the scope of theinvention.

FIG. 1A shows a schematic diagram of a drilling system 10 having abottom hole assembly (BHA) or drilling assembly 90 that includes sensorsfor downhole wellbore condition and location measurements. The BHA 90 isconveyed in a borehole 26. The drilling system 10 includes aconventional derrick 11 erected on a floor 12 which supports a rotarytable 14 that is rotated by a prime mover such as an electric motor (notshown) at a desired rotational speed. The drill string 20 includes atubing (drill pipe or coiled-tubing) 22 extending downward from thesurface into the borehole 26. A drill bit 50, attached to the drillstring 20 end, disintegrates the geological formations when it isrotated to drill the borehole 26. The drill string 20 is coupled to adrawworks 30 via a kelly joint 21, swivel 28 and line 29 through apulley (not shown). Drawworks 30 is operated to control the weight onbit (“WOB”), which is an important parameter that affects the rate ofpenetration (“ROP”). A tubing injector 14 a and a reel (not shown) areused instead of the rotary table 14 to inject the BHA into the wellborewhen a coiled-tubing is used as the conveying member 22. The operationsof the drawworks 30 and the tubing injector 14 a are known in the artand are thus not described in detail herein.

During drilling, a suitable drilling fluid 31 from a mud pit (source) 32is circulated under pressure through the drill string 20 by a mud pump34. The drilling fluid passes from the mud pump 34 into the drill string20 via a desurger 36 and the fluid line 38. The drilling fluid 31discharges at the borehole bottom 51 through openings in the drill bit50. The drilling fluid 31 circulates uphole through the annular space 27between the drill string 20 and the borehole 26 and returns to the mudpit 32 via a return line 35 and drill-cutting screen 85 that removes thedrill cuttings 86 from the returning drilling fluid 31 b. A sensor S₁ inline 38 provides information about the fluid flow rate. A surface torquesensor S₂ and a sensor S₃ associated with the drill string 20respectively provide information about the torque and the rotationalspeed of the drill string 20. Tubing injection speed is determined fromthe sensor S₅, while the sensor S₆ provides the hook load of the drillstring 20.

In some applications only rotating the drill pipe 22 rotates the drillbit 50. However, in many other applications, a downhole motor 55 (mudmotor) is disposed in the drilling assembly 90 to rotate the drill bit50 and the drill pipe 22 is rotated usually to supplement the rotationalpower, if required, and to effect changes in the drilling direction. Ineither case, the ROP for a given BHA largely depends on the WOB or thethrust force on the drill bit 50 and its rotational speed.

The mud motor 55 is coupled to the drill bit 50 via a drive disposed ina bearing assembly 57. The mud motor 55 rotates the drill bit 50 whenthe drilling fluid 31 passes through the mud motor 55 under pressure.The bearing assembly 57 supports the radial and axial forces of thedrill bit 50, the downthrust of the mud motor 55 and the reactive upwardloading from the applied weight on bit. A lower stabilizer 58 a coupledto the bearing assembly 57 acts as a centralizer for the lowermostportion of the drill string 20.

A surface control unit or processor 40 receives signals from thedownhole sensors and devices via a sensor 43 placed in the fluid line 38and signals from sensors S₁–S₆ and other sensors used in the system 10and processes such signals according to programmed instructions providedto the surface control unit 40. The surface control unit 40 displaysdesired drilling parameters and other information on a display/monitor42 that is utilized by an operator to control the drilling operations.The surface control unit 40 contains a computer, memory for storingdata, recorder for recording data and other peripherals. The surfacecontrol unit 40 also includes a simulation model and processes dataaccording to programmed instructions. The control unit 40 is preferablyadapted to activate alarms 44 when certain unsafe or undesirableoperating conditions occur.

The BHA may also contain formation evaluation sensors or devices fordetermining resistivity, density and porosity of the formationssurrounding the BHA. A gamma ray device for measuring the gamma rayintensity and other nuclear and non-nuclear devices used asmeasurement-while-drilling devices are suitably included in the BHA 90.As an example, FIG. 1A shows an example resistivity-measuring device 64in BHA 90. It provides signals from which resistivity of the formationnear or in front of the drill bit 50 is determined. The resistivitydevice 64 has transmitting antennae 66 a and 66 b spaced from thereceiving antennae 68 a and 68 b. In operation, the transmittedelectromagnetic waves are perturbed as they propagate through theformation surrounding the resistivity device 64. The receiving antennae68 a and 68 b detect the perturbed waves. Formation resistivity isderived from the phase and amplitude of the detected signals. Thedetected signals are processed by a downhole computer 70 to determinethe resistivity and dielectric values.

An inclinometer 74 and a gamma ray device 76 are suitably placed alongthe resistivity-measuring device 64 for respectively determining theinclination of the portion of the drill string near the drill bit 50 andthe formation gamma ray intensity. Any suitable inclinometer and gammaray device, however, may be utilized for the purposes of this invention.In addition, position sensors, such as accelerometers, magnetometers orgyroscopic devices may be disposed in the BHA to determine the drillstring azimuth, true coordinates and direction in the wellbore 26. Suchdevices are known in the art and are not described in detail herein.

In the above-described configuration, the mud motor 55 transfers powerto the drill bit 50 via one or more hollow shafts that run through theresistivity-measuring device 64. The hollow shaft enables the drillingfluid to pass from the mud motor 55 to the drill bit 50. In an alternateembodiment of the drill string 20, the mud motor 55 may be coupled belowresistivity measuring device 64 or at any other suitable place. Theabove described resistivity device, gamma ray device and theinclinometer are preferably placed in a common housing that may becoupled to the motor. The devices for measuring formation porosity,permeability and density (collectively designated by numeral 78) arepreferably placed above the mud motor 55. Such devices are known in theart and are thus not described in any detail.

As noted earlier, a significant portion of the current drilling systems,especially for drilling highly deviated and horizontal wellbores,utilize coiled-tubing for conveying the drilling assembly downhole. Insuch application a thruster 71 is deployed in the drill string 90 toprovide the required force on the drill bit. For the purpose of thisinvention, the term weight on bit is used to denote the force on the bitapplied to the drill bit during the drilling operation, whether appliedby adjusting the weight of the drill string or by thrusters. Also, whencoiled-tubing is utilized a rotary table does not rotate the tubing;instead it is injected into the wellbore by a suitable injector 14 awhile the downhole motor 55 rotates the drill bit 50.

FIG. 1B is a schematic view of an acoustic logging while drilling toolsystem on a BHA drill collar 90 containing a drill bit 50. This systemis mounted on the BHA drill collar 90 for performing acousticmeasurements while the formation is being drilled. The acoustic loggingwhile drilling tool system has a source 105 to emit acoustic vibrations106 that may traverse formation 95 and may also be propagated along theborehole wall and be received by sensors A and B which may be in arrays.These sensors are discussed later in the application. A point to note isthat the sensors are disposed between the transmitter and the receiver.This has important benefits in that the desired signal produced by thetransmitter travels in a direction opposite to the direction of noisegenerated by the drillbit 50. This makes it possible to use suitablefiltering techniques, including phased arrays, to greatly reduce thedrillbit noise. In an alternated embodiment of the invention, thetransmitter 105 may be located between the sensors and the drillbit 50

The application of the dipole acoustic technology to LWD has a seriousdrawback caused by the presence of the drilling collar with BHA thatoccupies a large part of the borehole. The drawback is that theformation dipole shear wave traveling along the borehole is severelycontaminated by the dipole wave traveling in the collar. This isdemonstrated by the following theoretical analysis/numerical modelingresults.

The dipole wave excitation and propagation characteristics for aborehole with a drilling collar are analyzed. Using known analysesmethods, for example the analyses of the type described in Schmitt(1988), one can calculate the velocity dispersion curve for theformation and collar dipole shear (flexural) waves. The dispersion curvedescribes the velocity variation of a wave mode with frequency. In theexample, the borehole diameter is 23.84 cm and the inner- and outerdiameter of the collar is 5.4 and 18 cm. respectively. The inner collarcolumn and the annulus column between the collar and borehole are filledwith drilling mud whose acoustic velocity and density are 1,470 m/s and1 g/cc, respectively. The collar is made of steel (compressionalvelocity, shear velocity and density of steel are 5,860 m/s, 3,130 m/s,and 7.85 g/cc, respectively). The formation is acoustically slow withcompressional velocity of 2,300 m/s, shear velocity 1,000 m/s, anddensity 2 g/cc. It is to be noted that the example is for illustrativepurposes only and not intended to be a limitation on the scope of theinvention.

The calculated drilling collar and formation flexural wave dispersioncurves for dipole modes are shown in FIG. 2, for the frequency rangeshown as the horizontal axis of 0 to 14 kHz. The collar dipole wavedispersion curve 201 displayed along the vertical axis shows howvelocity of the collar dipole wave varies with frequency over the range0 to 14 kHz. The formation dipole wave dispersion curve 203 shows thatexcept for low frequencies in this range, there is relatively littlechange in velocity. The formation and collar flexural wave modes coexistalmost for the entire frequency range, except at the very low frequencywhere the collar flexural mode appears to terminate at the formationshear velocity. Below the frequency where the collar mode terminates,the formation flexural mode velocity appears to continue the collarflexural mode behavior that would exist in the absence of the formation,the velocity decreasing to zero at the zero frequency. This cross-overphenomenon is caused by the strong acoustic interaction between thecollar and the formation in this dipole excitation situation.

Dipole acoustic wave propagation in a borehole with a drilling collarmay be simulated with numerical finite difference techniques, forexample as described in Cheng et al. (1995). The model parameters arethe same as used in the above velocity dispersion calculation example.The dipole source and an array of receivers are placed on the rim of thecollar. The source excitation center frequency is 2 kHz. FIG. 3 showsthe simulated dipole acoustic waveforms displayed with time along thehorizontal axis. Dipole acoustic waveforms are displayed by offsetcorresponding to source-receiver distance along the vertical axis inFIG. 3. Receiver locations are spaced 0.1524 m apart beginning with thenear trace waveform at 1.833 m 301 to 2.9 m 303 offset from the source.The dipole acoustic waveforms are dominated by the collar flexural wavethat has a faster velocity and a dispersive character. The initialstrong impulses 305 of the dispersive collar flexural wave are followedin time by dispersive energy that is stronger than the flexural wave ofinterest representing the formation S-wave. The dashed line 335represents the arrival location and velocity of the formation flexuralwave. The formation flexural wave of interest, aligned with dashed line335, is smaller in amplitude compared with the collar flexural wave, andis severely contaminated by the interaction of the collar flexural wave.

The above velocity dispersion calculation and wave propagationsimulation results demonstrate that there is a strong interactionbetween formation and drilling collar flexural waves. The fundamentalcause of this is that the two wave modes coexist in the same frequencyrange of interest. There are several drawbacks of the LWD dipoletechnology for shear velocity measurement. The first is the difficultyin obtaining good quality formation flexural wave measurements in thepresence of severe contamination from the collar flexural wave. Thecollar wave, being a low-frequency mode, is difficult to suppress usingattenuation techniques such as cutting grooves and/or insertingabsorptive materials in the collar, etc. The second drawback is that theformation flexural wave, even if it could be reliably measured, cannotyield accurate formation shear velocity information because of thecollar interaction. As can be seen from FIG. 2, the high frequency limitof the collar mode approaches the shear velocity of the collar and atthe frequency 205 where the collar dipole mode ceases to exist, thevelocity of the collar dipole mode is the formation shear velocity 207.The velocity 203 of the formation dipole mode is, at all frequencies,below the formation shear velocity 205. In the absence of a collar, whenexcited at low frequencies, the formation dipole mode would propagate ata velocity equal to the formation shear velocity. Determination of theformation shear velocity from the dispersion curves of either theformation dipole mode or the collar dipole mode not an easy matter. Inaddition, during drilling operations, there is a strong dipole modeexcited by the drillbit that produces additional noise. For theforegoing reasons, it is difficult, if not impossible, to measureformation shear wave velocity using the dipole acoustic technology inthe LWD environment.

The preferred manner of deploying certain sensors for LWD shear wavedata acquisition, for example in drill string BHA 90 of FIGS. 1A and 1Bwill now be described. The actual BHA utilized for a particularapplication may contain some or all of the sensors described in thebackground section, in addition to the present invention. For thepurpose of this invention any such BHA could contain one or more seismicsources and receivers (collectively represented herein by numeral 88) ata suitable location in the BHA 90.

This invention incorporates a quadrupole acoustic technology for the LWDshear velocity measurement. FIG. 4 a shows an illustration of aquadrupole field. It shows a characteristic four-lobed pattern that maybe substantially characterized by a cos 2θ azimuthal variation. In thepresent instance, the field may be associated with particle motion orwith pressure. The greatest advantage of using the quadrupole wave isthat the collar quadrupole wave exists only above a certain frequencycalled the cut-off frequency. This frequency depends on the drillingcollar thickness and borehole size. Thus, by determining the cut-offfrequency for the collar and borehole dimensions, one can select afrequency band in which only the formation quadrupole shear wave isexcited. Determining the velocity of the formation quadrupole wave willfulfill the needs for shear wave velocity measurement in the LWDenvironment. This is different from the situation in wireline logging:while Winbow does discuss the use of quadrupole logging on a wirelinedevice, there is no discussion of a cutoff frequency of a quadrupolewave. Winbow teaches a wireline logging device. As would be known tothose versed in the art, the tool diameter in wireline devices istypically much smaller than the borehole diameter. Chen has alsodiscussed the use of a quadrupole source for shear wave logging andshows a comparison of experimental results with theoretical dispersioncurves for Stoneley waves within the borehole. The tool diameter Chen'smodel is less than half the borehole diameter. As would be known tothose versed in the art, Stoneley waves are interface waves propagatingalong the wall of the borehole. In contrast, the present invention dealswith a MWD device wherein the diameter of the drill collar is almost thesame as the diameter of the borehole. In such a case, the collar modebecomes important. This collar mode is different from the Stoneley wavesdiscussed in Chen.

The feasibility of determining formation shear velocity from quadrupolewave measurement is demonstrated using theoretical/numerical analysisexamples. FIGS. 4–b shows the velocity dispersion curves of theformation 401 and collar quadrupole waves 403 and 405. Velocity in meterper second (m/s) is displayed along the vertical axis and frequency inkilohertz (kHz) along the horizontal axis. The velocity dispersion curvefor an exemplary collar of thickness 35 mm is shown as curve 403. Thevelocity dispersion curve for an exemplary collar of thickness 63 mm isshown as curve 405. The formation quadrupole wave is slightly dispersiveand reaches the formation shear wave velocity at a low cut-off frequency(around 2 kHz in this case). This indicates that formation shear wavevelocity can be determined as the low frequency limit of the velocity offormation quadrupole waves. The collar quadrupole wave velocity curveshows very high values due to the high shear rigidity (steel) and thickwall (63 mm) of the drilling collar. The collar wave for the 63 mm thickcollar 405, however, exists only-in the frequency range above 10 kHz;whereas, the required frequency for shear velocity measurement of theformation is around 2 kHz, well separated from the frequency range (>10kHz) of the collar wave. This frequency separation allows for designinga method and apparatus to generate quadrupole waves only in apredetermined frequency band (0–10 kHz in this case). In this band, onlythe formation quadrupole wave is generated. This waveexcitation/generation scheme may be demonstrated using finite differencesimulations.

In a finite difference modeling, a quadrupole source and an array ofreceivers with a 0.1524 m. spacing are placed at the rim of the collar.The source wavelet center frequency is chosen to be 2 kHz, the amplitudespectrum of the source wavelet diminishing to zero at about 5 kHz, whichis below 10 kHz, the cut-off frequency of the collar quadrupole wave.FIG. 5 a shows three snapshots in time of the wavefield intensity due tothe quadrupole source. Each snapshot, 501, 503, and 505 is across-section displaying a fluid inside the drill collar 521, the drillcollar 90, fluid 523 outside the drill collar, and a formation 95.Radial particle velocity associated with the quadrupole wave motion isdisplayed shaded 507, 509. These FIG. 5 a snapshots display thelocations of wavefield intensity at several instants of time (0.24milliseconds for 501, 1.22 milliseconds for 503, and 2.21 millisecondsfor 505, respectively). In the borehole fluid annulus high wavefieldintensity is seen 507 and high wavefield intensity is seen in theformation 509. The wave motion in the fluid annulus and the motion inthe formation are in phase, traveling at the formation shear velocity asdemonstrated by the juxtaposition of high wavefield intensity in thefluid 507 and the formation 509 for all three snapshots 501, 503 and505. This is the formation quadrupole wave as discussed in connectionwith FIG. 4. There is almost no discernable wave intensity for locationsanywhere inside the collar except for the initial time (snapshot 0.24 ms501) around the source, showing that the collar quadrupole wave is notexcited in this frequency band for this collar thickness (2.48 inches).

The time-domain quadrupole waveforms of the receiver array are displayedin FIG. 5B where time is represented along the horizontal axis.Quadrupole acoustic waveforms are displayed by offset corresponding tosource-receiver distance along the vertical axis in FIG. 5B. Receiverlocations are spaced 0.1524 m apart beginning with the near tracewaveform at 1.833 m 531 to 2.9 m 533 offset from the source. FIG. 5Bshows only one wave, the formation quadrupole wave, traveling across thearray at the formation shear velocity. The dashed line 535 marks theformation quadrupole wave first arrival. This time-domain wavesimulation result is consistent with the frequency domain dispersionanalysis result of FIG. 4. These examples demonstrate that choosing anappropriate wave excitation frequency band can eliminate collarquadrupole waves.

The excitation frequency band should be selected based on the drillingcollar and borehole dimensions. In drilling practice, the size of theborehole is varied to suit exploration/production needs and/orgeological environment of the well to be drilled. Accordingly, the sizeof the drilling collar also varies. The change of the collar andborehole dimensions, especially the former, will significantly changethe collar quadrupole wave characteristics. This, again, can bedemonstrated using theoretical/numerical analysis examples. Thequadrupole dispersion curves for a 35 mm thickness drilling collar(dashed curves) are shown in FIG. 4. With the thinner collar, theformation quadrupole dispersion curve (dashed line 403) is also changed.The cut-off frequency of the formation mode is raised to about 3 kHz andthe whole curve exhibits higher values compared to the thick collar case(solid curve 405). The curve approaches the formation shear wavevelocity at low frequencies. On the other hand, the collar quadrupolewave velocity is much reduced compared to the thick collar case (solidcurve 405) and shows a much lower cut-off frequency (about 6 kHz). Thegeneral trend shown in FIG. 4 is that increasing the collar thicknesswill move the frequency band of the collar quadrupole wave away fromthat of the formation quadrupole wave. When the frequency band of thecollar quadrupole wave is close to that of the formation quadrupole,measurement of the formation quadrupole wave will be severely impaired,as the following wave simulation example demonstrates.

The example case is where the collar pipe is relatively thin (thicknessis only 16 mm or 0.63 inches). The cut-off frequency of the collarquadrupole wave is now about 3.5 kHz. Other parameters, as used in thesimulation of FIG. 5B, are kept unchanged. With the 2 kHz centerexcitation frequency, both formation and collar quadrupole waves areexcited. FIG. 6A is the counterpart of FIG. 5A, showing the wavefieldsnapshots for the 16 mm thick collar case. FIG. 6A shows three snapshotsin time of the wavefield intensity due to the quadrupole source. Eachsnapshot, 601, 603, and 605 is a cross-section displaying a fluid 621inside the drill collar, drill collar 90, fluid 625 outside the drillcollar, and a formation 95. Radial particle velocity associated with thequadrupole wave motion is displayed shaded 611, 613. These FIG. 6Asnapshots display the locations of wavefield intensity at severalinstants of time (0.24 milliseconds for 601, 1.22 milliseconds for 603,and 2.21 milliseconds for 605, respectively). In the borehole fluid 625annulus high wavefield intensity is seen 611 and high wavefieldintensity is seen in the collar 613. For all the three time instants601, 603 and 605, the wavefield intensity in the entire boreholecontaining the collar is dominated by the collar quadrupole wave.Formation quadrupole wave is weakly excited, which is out of phase withthe collar wave.

The time domain array waveforms are shown in FIG. 6B. The time-domainquadrupole waveforms of the receiver array are displayed in FIG. 6Bwhere time is represented along the horizontal axis. Quadrupole acousticwaveforms are displayed by offset corresponding to source-receiverdistance along the vertical axis in FIG. 6B. Receiver locations arespaced 0.1524 m apart beginning with the near trace waveform at 1.833 m631 to 2.9 m 633 offset from the source. The time domain waveforms aredominated by the collar quadrupole wave that has a moveout velocity ofabout 1,300 m/s. The dashed line 635 represents the arrival time for theformation quadrupole wave. In this case, it is difficult to measure theformation quadrupole wave in the presence of the strong collar wave.This example demonstrates the importance of the frequency band selectionfor the LWD quadrupole wave generation and measurement devices.

The above examples demonstrate the advantage of the quadrupole shearwave in the LWD environment and the requirements that facilitate makingthe LWD quadrupole shear wave measurement. As a result, a quadrupole LWDshear wave system can be constructed. Quadrupole shear waves in the LWDenvironment can be generated using a quadrupole source mounted on thedrilling collar 90. FIG. 7 is a simplified perspective view of thesource cross-section in conjunction with the drilling collar. As shownin FIG. 7, the quadrupole source comprises the drilling collar 90 andeight members of equal dimension. The sections are number 701–708. Thesemembers are eight equal sectors of the source cylinder. The cylindersections are made from either an electrostrictive (or piezoelectric) ora magnetostrictive material capable of generating stress/pressure wavesignals from the input electric pulse. In an alternate embodiment of theinvention (not shown) the sections comprise electromechanical devices ofsuch devices are shown in U.S. Pat. No. 5,852,262 to Gill, the contentsof which are incorporated herein by reference. Included in the teachingsof Gill is an actuator that produces a pressure pulse. By use ofsuitably configured portholes, dipole or quadrupole pulses may beproduced. Bender bars may also be used. Although dividing the sourcecylinder into four equal sectors suffices to produce a quadrupolesource, using eight (or any multiple of four) sectors for the sourcereduces the mass of each sector so they more easily withstand drillingvibrations. While the description of the source herein uses eight sourcesegments as an example, those versed in the art would recognize how anymultiple of four sources could be excited to produce a quadrupolesignal.

The lower part of FIG. 7 is a cross-sectional view of the quadrupoleshear wave source on the plane perpendicular to the axis of the drillingcollar. The elements of the source device are, in one embodiment, eightsectors labeled 701, 702, 703, 704, 705, 706, 707 and 708. Whenelectrical pulses are applied to the source, each sector will expand orcontract in a radially outward or inward manner. Specifically, theelectrical pulses can be applied such that sectors (701, 702) anddiametrically opposed sectors (705, 706) will expand and simultaneously,sectors (703, 704) and sectors (707, 708) will contract, as illustratedin FIG. 7. Then four stress/pressure waves will be generated in thesurrounding borehole fluid/formation, as well as in the drilling collar.It is also to be noted that when a device such as that taught in Gill isused, there may only be a single actuator that produces quadrupolesignals from suitable portholes.

When all eight sectors are made from the same material and theelectrical pulses applied to them have substantially the same amplitude,then the interaction of the four pressure/stress waves inside thedrilling collar and in the surrounding borehole/formation will producequadrupole shear waves. More specifically, if the electrical pulses aremodulated such that the frequency band of the generated pressure/stresswaves is below the cut-off frequency of the quadrupole shear wave in thedrilling collar, then the interaction of the four stress waves in thecollar will cancel each other. The interaction of the pressure/stresswave in the borehole and formation will produce a formation quadrupoleshear wave to propagate longitudinally along the borehole. Thisfrequency band modulation of the source pulses is part of one embodimentof the present invention.

An actual implementation of the quadrupole shear wave source is nowdescribed in more detail. FIG. 8 illustrates a detailed cross-sectionalview of a segmented ring transmitter implementation. This implementationhas a segmented ring mounted on an elastomeric isolation component 801in a continuous pocket 803 around the collar 90. The isolation componentis used to reduce drill string induced acoustic signals. The sensor subassembly can be oil-filled with a cover sleeve 807, as illustrated, orit can be a potted assembly, requiring no oil. An electronic drivecircuit(s) 809 is coupled to the transmitter element(s) 811 to exciteacoustic signal(s) into the borehole and formation. A cover sleeve 815may cover transmitter elements 811. The electronic drive circuit 809 maybe placed in the inner mandrel 813. The transmitting element(s) 811 canbe implemented in various forms to include rings, cylinders, plates, andpiston or solenoid elements. The number of segments can be increased incircumferential and/or axial direction as needed.

FIG. 9 illustrates a detailed cross-sectional view of acompartmentalized segmented transmitting implementation which hassegments mounted on individual elastomeric isolation components 901 incompartmentalized pockets 903 around the collar 90. There may be oilfill 907 between the transmitting element 911 and a cover sleeve 915.The transmitting electronics 909 may reside in the inner mandrel 813.

The quadrupole shear wave in the earth formation produced by the abovedescribed quadrupole source may be detected at locations spacedlongitudinally along the borehole from the source, as schematicallyillustrated in FIG. 1B. FIG. 1B is a schematic view of an acousticlogging while drilling tool system on a BHA drill collar 90 containing adrill bit 50. This system is mounted on the BHA drill collar 90 forperforming acoustic measurements while the formation is being drilled.The acoustic logging while drilling tool system may have a source 105 toemit acoustic vibrations 106 that may traverse formation 95 and bereceived by sensors A and B which may be in arrays. The use of arrayswith axially spaced apart elements as shown in FIG. 1B makes it possibleto use well-known phased array filtering techniques for attenuatingsignals traveling in a particular direction with a specified range ofvelocities. In a preferred embodiment of the invention, this phasedarray filtering is used to attenuate noise generated by the borehole.The phased array filtering may also be used to attenuate the collar modeby defining a range of velocities to be attenuated. In an alternateembodiment of the invention, a plurality of axially spaced aparttransmitter elements is used and the phased array filtering is used toreduce the generation of collar mode signals.

As shown in FIG. 10 for an embodiment of the present invention, acousticdetectors may be mounted at the rim of the drilling collar. Eachdetector consists of four similar sensors circumferentially spacedaround the collar, for example sequentially named as A, B, C, and D.FIG. 10 is a perspective view of a sensor layout around the collar inrelation to source elements as described in FIG. 7. The center of eachsensor is longitudinally aligned with a junction of two source sectorsthat expand/contract simultaneously. For example, sensor A is alignedwith the junction between source sectors 701 and 702, sensor B isaligned with the junction between source sectors 703 and 704, etc. Thefour-sensor detector acts to enhance the efficiency of the quadrupolesource and to remove possible contamination from a monopole component ofthe wave signal.

An actual implementation of the quadrupole shear wave receivers asquadrupole detector arrays is now described in more detail. FIG. 11illustrates a detailed cross-sectional view of a segmented ringreceiving sensor implementation which has a segmented ring mounted on anelastomeric isolation component 1101 in a continuous pocket 1103 aroundthe collar 90. The isolation is used to reduce drill string inducedsignals. The sensor sub assembly can be oil-filled 1107 with a coversleeve 1115, as illustrated or it can be a potted assembly, requiring nooil. An electronic signal conditioning circuit(s) 1109 is coupled to thereceiver element(s) 1111 to sense acoustic signal(s) in the borehole andformation. The receiver circuit 1109 may reside within the inner mandrel813. The receiving element(s) can be implemented in various forms toinclude rings, cylinders, plates, and piston or solenoid elements. Thenumber of segments can be increased in a circumferential and/or axialdirection as needed. The material for the receiver elements may beselected from a piezoelectric material, an electrostrictive material;and, a magnetostrictive material.

FIG. 12 illustrates a detailed cross-sectional view of acompartmentalized segmented receiving sensor implementation which hasreceiver segments 1211 mounted on individual elastomeric isolationcomponents 1201 in compartmentalized pockets 1203 around the collar 90.The inner mandrel 813 holds receiver circuit 1209, which is coupled witha receiver element 1211. The receiver element 1211 has a cover sleeve1215 with an oil filled space 1207 in between.

Copending U.S. patent application Ser. No. 09/590,906, the contents ofwhich are fully incorporated herein by reference, teaches how aquadrupole mode may obtained by suitable combination of signals receivedby two receiver elements with two different excitations of a quadrupoletransmitter.

This is illustrated in FIG. 13 a showing another embodiment of theinvention in which a quadrupole excitation is provided by four segments1401 a–1401 d of a transmitter Alternatively, the quadrupole signalcould be provided by pairing adjacent segments of the eight-elementtransmitter discussed above. Hereafter, when references are made to fourelement transmitters and receivers, it is to be understood that theseare intended to cover eight element transmitters and receivers as well.The signal, after propagating through the formation, is detected by adipole receiver 1410 comprising receiver elements 1403 a and 1403 c.Subsequently, a quadrupole mode is excited as in FIG. 13 b by thetransmitter 1400 and received by the same receiver elements 1403 a and1403 c. Denoting by A and C the signals received by the receiverelements 1403 a and 1403 b for the transmitter firing of FIG. 13 a, andby A* and C* the signals received by the receiver elements 1403 a and1403 b in FIG. 13 b, by performing the operationS=(A+C)−(A*+C*),the signal S will be a quadrupole signal with all common modes includingthe BHA/drillbit signals attenuated. These common modes include the bodywave through the tool, monopole signals (P and S) through the formationand Stoneley waves within the borehole. The Stoneley waves are more of aproblem with an MWD tool than with a wireline tool due to the smallerannulus between the tool and the borehole wall: consequently, removal ofthe common modes is more important than for wireline logging.

FIG. 14 shows a side view of a receiving sensor segment. The collar 90contains a transducer element 1211 that includes a plurality of activeelements 1251. The other components in FIG. 14 are as in FIG. 12 and arenot discussed further.

The signals received from, for example, the FIG. 10 sensors A, B, C, andD may be combined as (A+C)−(B+D). For the source configuration andfiring scheme shown in FIG. 7, the generated quadrupole wave arriving atthe sensors A, B, C, and D can be described as follows. Sensors A and Cwill see, respectively, a pressure wave of the same polarity andamplitude, and sensors B and D, a pressure wave of the same amplitudebut reversed polarity. Therefore, adding the signals from A and C andsubtracting the signals from B and D, as (A+C)−(B+D), will produce asignal four times stronger than each individual signal of one sensor.Moreover, a LWD quadrupole source, when placed off the borehole axis, asis often the case during drilling, will also generate a monopolecomponent to propagate along the borehole. At low frequencies, themonopole wave will propagate as a tube wave to contaminate the formationquadrupole shear wave. The low frequency tube wave motion issubstantially homogeneous inside the borehole, such that each individualsensor will measure substantially the same tube wave signal. Thismonopole tube wave can be canceled out by the combination (A+C)−(B+D).

An alternative embodiment of the invention is the low-pass filtering ofthe signals recorded by each sensor below the cut-off frequency of thecollar quadrupole wave. If the source pulses have high frequencies andare not modulated, or if the quadrupole component of the drilling noisehas frequency content above the cut-off frequency of the collarquadrupole wave, then the collar quadrupole wave will be excited andrecorded by the sensors. Because the collar quadrupole wave exists onlyabove its cut-off frequency, it can be removed by low-pass filtering thesensor signals below the cut-off frequency. As stated above, thiscut-off frequency can be determined from the borehole and drillingcollar dimensions. Thus the low-pass filter can be designed andinstalled as a part of the sensor data acquisition and processingsystem.

For signal coherence enhancement and processing purposes, arrays of theabove-mentioned detectors/receivers are placed longitudinally along thedrilling collar. The spacing between the adjacent detectors typicallyranges from 0.5 to 1 ft (0.1524 to 0.3048 m). The formation shear wavevelocity may be determined by correlating the quadrupole shear waveacross the receiver array using any standard array processing method astaught in U.S. Pat. No. 5,441,890 to Tang.

The above discussion has been directed towards determination offormation shear velocities by what would be recognized by those versedin the art as the first order mode. In “fast” formations where the shearvelocity exceeds the compressional velocity in the borehole fluid,higher modes may exist. This is illustrated in FIG. 15 a. The abscissais-the frequency and the ordinate is the velocity. 1301 a and 1301 bshow an example of the phase and group velocities of the 1^(st) orderquadrupole mode while 1303 a and 1303 b show the phase and groupvelocities of the 2^(nd) order mode. The formation shear- andcompressional-wave velocities are 4,500 and 2,500 m/s, respectively; thedrilling collar and borehole dimensions and other related parameters arethe same as in FIG. 5B. The waveform simulation uses a 5-kHzcenter-frequency source. Two quadrupole modes exist in the frequencyrange below 21 kHz. They are named the first and the second quadrupolemode, respectively, with the higher order mode existing in a higherfrequency range. For the example in FIG. 15A, the first and the secondmode starts to appear from about 3 and 8 kHz, respectively. The firstmode shows a steeply decreasing phase dispersion curve 1301 a with awell-defined group velocity 1301 b minimum. The presence of the groupvelocity minimum produces a phenomenon called the Airy phase. Theassociated waveforms are shown in FIG. 15B. The wave energy associatedwith the Airy phase dominates the waveforms and lags significantlybehind the formation shear-arrival time 1311, as shown in FIG. 15B. Inthis situation, it is difficult, if not impossible, to measure the fastformation shear-wave velocity from the waveform data for the first mode.

Measuring the waveform of the second quadrupole wave mode can directlydetermine the fast formation shear velocity. As shown in FIG. 15A, in abroad frequency range above the cut-off frequency, the phase velocitycurve of the second mode is very close to the formation shear velocity(2,500 m/s in this case). Both the phase and group velocity curves ofthe second mode are quite flat, indicating minimal dispersion effects.In fact, the quadrupole wave excited in this higher frequency range,which is associated with the second mode, propagates at the formationshear-wave velocity, as illustrated in FIG. 15B. Since the secondquadrupole mode of the formation can be effectively excited below thefrequency range of the collar quadrupole, the interference from thelatter mode can still be avoided or minimized. As shown in FIG. 15B,there is no discernable wave energy associated with the collarquadrupole. The example shown in FIGS. 15A, 15B shows that one canreliably determine the formation shear-wave velocity by exciting andmeasuring the second quadrupole wave mode associated with a fastformation.

The above examples have used a unitary drill collar to illustrate theprinciples of using quadrupole mode excitation for determination offormation shear properties in a LWD environment. For practical reasons,the logging tool of the present invention may comprise multiple segmentsof drill collars with transmitters and receivers on different segmentsof the drill collar. The principles described above are equallyapplicable to such segmented drill collars and are intended to be withinthe scope of the invention. It is possible that the cut-off frequencyand the dispersion curves may be different for each segment of the drillcollar. This does not affect the principles of operation of the loggingtool.

The above examples have further illustrated the implementation ofquadrupoles by at least two pairs of matched “monopole” elements whereinelements within each pair move in unison. An alternative method forexcitation of a quadrupole signal or reception of a quadrupole signal ispossible using two dipoles, such as bender bars. This is illustrated inFIG. 16. The elements 1401–1403 comprise one dipole, e.g., a firstbender bar, and the elements 1405–1407 comprise a second dipole, e.g., asecond bendor bar. The motion of the bender bars is not radial: it isorthogonal to a radial line joining the center of the tool to themidpoint of the bender bar; nevertheless, a significant quadrupolesignal may be excited or received by the configuration shown in FIG. 16.Such an implementation is intended to be within the scope of the presentapplication.

The above description has been made with respect to a LWD application ofthe invention. The present invention may also be used inLogging-while-Tripping. As would be known to those versed in the art, itis common for drillbits to wear out and the process of removing adrillstring for the purpose of changing a drill bit is called“tripping.” Making measurements while tripping has the additionaladvantage that no drilling is being carried out, so that the signalsreceived by the receivers of the present logging tool are notcontaminated by drillbit generated noise.

While the foregoing disclosure is directed to the preferred embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all variations within the scopeand spirit of the appended claims be embraced by the foregoingdisclosure.

1. An acoustic logging apparatus comprising: (a) a drill collar conveyedon a drilling tubular in a borehole within a formation, said drillcollar having a cutoff frequency for a collar mode wave therein; (b) anacoustic transmitter on the collar producing a signal, said signalcomprising a formation mode and a collar mode; (c) at least one signaldetector on the drill collar for detecting said signal, said at leastone signal detector spaced apart from the transmitter in an axialdirection of the collar and receiving signals including the formationmode and the collar mode; and (d) a processor including a filter forlow-pass filtering of a component of the received signals having afrequency below the cutoff frequency.
 2. The logging apparatus of claim1 wherein said collar mode is a quadrupole mode and said transmitter isa quadrupole transmitter.
 3. The logging apparatus of claim 1 whereinthe at least one signal detector further comprises a plurality ofdetector elements disposed circumferentially about the collar.
 4. Thelogging apparatus of claim 3 wherein the transmitter further comprises aplurality of transmitter elements.
 5. The logging apparatus of claim 2wherein said quadrupole transmitter further comprises at least 2N pairsof diametrically opposed transmitter elements disposed circumferentiallyaround said collar, where N is an integer.
 6. The logging apparatus ofclaim 4 wherein said transmitter elements further comprise a materialselected from: (i) a piezoelectric material, (ii) an electrostrictivematerial; and, (iii) a magnetostrictive material.
 7. The loggingapparatus of claim 4 wherein said transmitter elements further comprisea device selected from (i) a bender bar, (ii) an electromechanicaldevice, and, (iii) a porthole.
 8. The logging apparatus of claim 1wherein the at least one detector further comprises a plurality ofdetector elements comprising a material selected from: (i) apiezoelectric material, (ii) an electrostrictive material; and, (iii) amagnetostrictive material.
 9. The logging apparatus of claim 1 whereinsaid formation has a shear velocity greater than a compressionalvelocity of a fluid in the borehole and said signal further comprises afirst quadrupole mode and a second quadrupole mode, and said processorfurther comprises a filter for high pass filtering said signal above anAiry phase associated with the first quadrupole mode.
 10. A shear wavelogging apparatus comprising: (a) a drilling collar conveyed on adrilling tubular in a borehole within a formation, said drilling collarhaving a cutoff frequency for a collar mode wave therein; (b) aquadrupole transmitter on the collar producing a signal, said signalcomprising a formation mode and a collar mode; (c) at least one detectorspaced axially apart from the quadrupole transmitter for detecting saidsignal; (d) a processor for processing the detected signal using afilter for low pass filtering components of the signal below said cutofffrequency and determining therefrom a shear velocity of the formation.11. A method of using an acoustic logging apparatus on drilling collarconveyed on a drilling tubular in a borehole within a formation, themethod comprising: (a) using an acoustic transmitter on the loggingapparatus for producing a quadrupole signal comprising a formation modeand a tool mode; (b) using at least one signal detector on the drillingcollar spaced apart axially from the transmitter for detecting saidsignal; and (d) using a processor for low-pass filtering a component ofthe detected signal having a frequency below a cutoff frequency of thetool mode in the drill collar.
 12. The method of claim 11 wherein saidtransmitter is comprises two pairs of diametrically opposed transmitterelements and producing said signal further comprises: activating saidelements to produce a signal having a cos 2θ azimuthal variation. 13.The method of claim 11 wherein said transmitter further comprises twodipoles.
 14. The method of claim 11 wherein the at least one signaldetector further comprises detector elements disposed circumferentiallyon the collar.
 15. The method of claim 11 wherein said at least onesignal detector further comprises a plurality of axially spaced-apartsignal detectors.
 16. The method of claim 11 wherein the at least onetransmitter comprises a quadrupole transmitter and the at least onesignal detector comprises two detector elements, the method furthercomprising operating the transmitter at a first time with onepolarization and at a second time with a second polarization.
 17. Themethod of claim 15 wherein processing said signal further comprisesusing said plurality of axially spaced-apart detectors for beamsteering.
 18. The method of claim 11 wherein said formation has a shearvelocity greater than a compressional velocity of a fluid in theborehole and said signal further comprises a first quadrupole mode and asecond quadrupole mode, the method further comprising using saidprocessor for high pass filtering said signal above an Airy phaseassociated with the first quadrupole mode.
 19. A method of determining aparameter of interest of an earth formation using a shear wave loggingapparatus on a drilling collar, the method comprising: (a) using aquadrupole transmitter on the collar for producing a signal, said signalcomprising a formation mode and a tool mode; (c) using at least onedetector spaced axially apart from the quadrupole transmitter fordetecting said signal; (d) using a processor for processing the detectedsignal using a filter for attenuating components of the signal abovesaid cutoff frequency and determining therefrom a shear velocity of theformation.